Giant planet
A giant planet is any planet or exoplanet that is much larger than Earth . Giant planets typically consist primarily of substances with a low condensation point and less of rocks and metals. However, there are also solid giant planets. In the solar system , there are four known giant planets: Jupiter , Saturn , Uranus and Neptune . When searching for the hypothetical planet nine , one also assumes a giant planet. Many giant planets have been found among the previously known exoplanets orbiting other stars .
Non-solid giant planets are also called gas giants . While this term included all four giant planets of the solar system for a long time, many astronomers today only apply the term gas giant to Jupiter and Saturn and classify Uranus and Neptune, which have a different composition, as ice giants . Jupiter and Saturn are mostly made up of hydrogen and helium , while Uranus and Neptune are mostly made up of water , ammonia, and methane . Both names, gas giants and ice giants, are potentially ambiguous: the planets of both types are mainly composed of substances that are in a state of high pressure and high temperature above their respective critical points where there is no distinction between the liquid and gaseous phases and instead there is a hot supercritical fluid .
Another name for non-solid giant planets is Jovian planets . The adjective Jovian means "similar to Jupiter".
Demarcation
There is no fixed criterion from what size an (exo-) planet is considered a giant planet. There is agreement among the eight known planets of the solar system that the four outer ones fall into this category and the four inner ones do not.
Gas giants can be more massive than Jupiter. They are then called Super Jupiter . However, they cannot become significantly larger in radius or diameter than Jupiter, because with increasing mass they compress due to the stronger gravitational effect in such a way that their density continues to increase and their radius hardly increases. Above a certain limit, which is approximately 13 Jupiter's masses, deuterium fuses inside . One then no longer speaks of (exo-) planets, but of brown dwarfs . However, even below the deuterium fusion, some astronomers already speak of brown dwarfs of low mass or sub- brown dwarfs . There is no recognized criterion for delimiting giant planets or massive objects of planetary mass and these. According to one approach, they are differentiated according to their type of origin ( origin criterion ), according to another approach according to the physical processes that prevail in them ( fusion criterion ).
terminology
The term gas giant was coined in 1952 by science fiction author James Blish and originally referred to all four giant planets known at the time. “Gas giant” is a bit misleading, because in most of these planets the temperature and density are so high that the matter is not in the gaseous state there. Apart from the gaseous upper layers of the atmosphere and a possible solid core, all matter in such a planet is in the supercritical state, where there is no difference between liquid and gas. Fluid planet would therefore be a more appropriate term.
The term gas giant has nevertheless established itself because it fits together with a special language used by planetologists . Planetologists use the category terms "rock", "ice" and "gases" to classify elements and compounds from which planets are predominantly, regardless of in which state of aggregation they are actually present. Silicates and metals are called "rocks", water, ammonia and methane are called "ice", and hydrogen and helium are called "gases". So this is a different usage than the usual meanings of the words rock , ice and gas . Because of this usage, some astronomers began to refer to the planets Uranus and Neptune as ice giants , because it became known that they only consist of "gases" (in the planetological sense) on the outside, but not primarily.
Subtypes
Gas giants
Gas giants are mainly made up of hydrogen and helium. The solar system's gas giants, Jupiter and Saturn, also contain heavier elements that make up between 3 and 13 percent of their mass.
Gas giants are believed to have a mantle of molecular hydrogen that surrounds an inner layer of liquid metallic hydrogen that makes up most of the planet. This high pressure modification of hydrogen is called “metallic” because it is electrically conductive . Even deeper, there is possibly a molten rock core made of heavier elements, which is exposed to such high pressures and temperatures that its properties are still poorly understood.
Also Hot Jupiter among the gas giants.
Ice giants
Ice giants are smaller, less massive and have a significantly different structure than gas giants. The ice giants of the solar system, Uranus and Neptune, have a hydrogen-rich atmosphere that extends from the upper cloud layers down to about 80% (Uranus) or 85% (Neptune) of the planet's radius. Below that, they mainly consist of water, methane and ammonia, ie substances that are planetologically categorized as "ice". They also contain “rocks” and “gases” in the deeper layers, but the exact composition is still unknown.
The atmospheres of Uranus and Neptune are foggy and colored blue-green by small amounts of methane. Both planets have magnetic fields that are clearly inclined against their axis of rotation. Uranus has more hydrogen and helium than Neptune, but is overall less massive. Neptune is thus denser; it also has significantly higher internal temperatures and a much more active atmosphere with high wind speeds. According to the Nice model , Neptune originally evolved closer to the sun than Uranus and should accordingly contain more heavy elements.
All four known gas giants and ice giants of the solar system have ring systems made of ice or rock particles with many moons , whereby the rings of Saturn are most clearly formed, followed by the rings of Uranus .
Also hot Neptunes among the ice giants.
Very large rock planets
There are also very large rock planets , for example Kepler-10c . This was proposed as a prototype for a new class of planets that is colloquially called Mega Earth; an increase in the colloquial term super earth .
Rocky planets with up to thousands of earth masses could possibly form around massive stars ( B-stars and O-stars with 5 to 120 solar masses) if their protoplanetary disk contains sufficiently heavy elements when the star system is formed. Such stars could also have enough UV radiation and stellar winds to deprive their planets of their atmospheres by means of photoevaporation , so that only the heavy elements remain.
Super puff
A super-puff is an exoplanet with a mass only a few times greater than that of Earth , but a radius greater than Neptune . So it has a very low average density. Super puffs are cooler and less massive than the Hot Jupiter with their thermally expanded atmospheres.
The most extreme known examples are the three planets around Kepler-51 , all of which are Jupiter-sized.
See also
Individual evidence
- ↑ Planets and Exoplanets. In: uni-bonn.de. astro.uni-bonn.de, accessed on February 10, 2020 .
- ↑ Jonathan I. Lunine: The Atmospheres of Uranus and Neptune . In: Annual Review of Astronomy and Astrophysics . 31, September 1993, pp. 217-263. bibcode : 1993ARA & A..31..217L . doi : 10.1146 / annurev.aa.31.090193.001245 .
- ↑ a b A. J. Burgasser: Brown dwarfs: Failed stars, super Jupiters ( PDF ) In: Physics Today . June 2008. Retrieved January 11, 2016.
- ↑ G. D'Angelo, Durisen, RH, Lissauer, JJ: Giant Planet Formation . In: S. Seager. (Ed.): Exoplanets . University of Arizona Press, Tucson, AZ, 2011, pp. 319-346, arxiv : 1006.5486 .
- ↑ Jack J. Lissauer, David J. Stevenson: Formation of Giant Planets (PDF) In: NASA Ames Research Center; California Institute of Technology . 2006. Retrieved January 16, 2006.
- ↑ a b The Interior of Jupiter, Guillot et al., In Jupiter: The Planet, Satellites and Magnetosphere , Bagenal et al., Editors, Cambridge University Press, 2004
- ^ L. McFadden, P. Weissman, T. Johnson: Encyclopedia of the Solar System (2nd ed.) . Academic Press , 2007, ISBN 978-0-12-088589-3 .
- ^ S. Seager, M. Kuchner, CA Hier-Majumder, B. Militzer: Mass-Radius Relationships for Solid Exoplanets . In: The Astrophysical Journal . 669, No. 2, 2007, pp. 1279-1297. arxiv : 0707.2895 . bibcode : 2007ApJ ... 669.1279S . doi : 10.1086 / 521346 .
- ↑ a b c The Featureless Transmission Spectra of Two Super-Puff Planets , Jessica E. Libby-Roberts, Zachory K. Berta-Thompson, Jean-Michel Desert, Kento Masuda, Caroline V. Morley, Eric D. Lopez, Katherine M. Deck, Daniel Fabrycky, Jonathan J. Fortney, Michael R. Line, Roberto Sanchis-Ojeda, Joshua N. Winn, 28 Oct 2019